HCN ion channels and pain

Pain is critical for survival, as shown by the bodily damage and short lives of the rare humans who lack a sensation of pain. Pain also has a downside, however, because chronic pain causes severe limitations in mobility and quality of life of those who suffer from it. Three modes of pain can be distinguished: acute pain, caused by an initial injury; inflammatory pain, in which tissue damage releases inflammatory mediators which enhance the sensation of pain; and neuropathic pain, caused by direct nerve damage. Acute pain is essential to protect us from injury. Inflammatory pain can be beneficial in protecting damaged areas from further harm, but when chronic it can cause a major reduction in quality of life. Neuropathic pain serves no obvious protective function and is a severe handicap for those who suffer from it. It is also the most poorly understood mode of pain, and the most difficult to treat.
Recent work in the applicant laboratory has found that a single ion channel, HCN2, which is expressed in the surface membranes of nociceptive (pain-sensitive) neurons, controls their rate of firing of action potentials in response to inflammatory mediators. Selective genetic deletion of HCN2 in a subset of nociceptors abolishes the heat sensitivity seen in inflammatory pain, and also abolishes neuropathic pain. This work showed for the first time that a single molecular entity underlies both forms of pain, and moreover it opens up possibilities for new treatments of pain by the development of selective blockers of HCN2. This last will be a particularly welcome development, as all current analgesics in common use have major side effects.
The present application aims to build on this advance by extending our scientific understanding of the involvement of HCN2 in pain. We have identified a particular population of nociceptors where the expression of HCN2 is critical for pain. The method that we used to achieve the genetic deletion tells us that these neurons must express a voltage-dependent sodium channel, NaV1.8. What are the other electrophysiological and histological properties of these neurons that make them critical for pain? We will mark these neurons genetically and will isolate them for further study.
In two further lines of enquiry we will investigate which inflammatory mediators are crucial for modulating HCN2. Our previous work has identified prostaglandin E2 as one such mediator, but we have strong indications that others are important, in particular for neuropathic pain. We will test candidate mediators in isolated neurons, and we will elucidate their intracellular signalling pathways. We will then test inhibitors of these mediators, or of their signalling pathways, in in vivo experiments. This work may open up therapeutic possibilities for controlling pain by attacking the modulation of HCN2 rather than directly blocking the channel.
A fourth area will investigate how the expression of HCN2 in neurons, and its expression in the neuronal membrane itself, may be modulated in a more long-term way by inflammatory mediators of the growth factor family, such as NGF and GDNF. There is evidence that these factors are released following nerve injury, and they may be important in the long-term maintenance of neuropathic pain.
The work outlined above will put our understanding of the role of HCN2 in pain ona firm basis. There are two other members of the HCN ion channel family, HCN3 and HCN4, which remain to be investigated (previous work in our group has already ruled out HCN1 as a major contributor to pain). In the final phase of the project we will construct animals with genetic deletions of these two ion channels and will examine the effect of the deletion on pain. This work will tell us in a straightforward fashion whether either channel plays an important role in either inflammatory or neuropathic pain.

Recent work in the applicant lab has shown that the HCN (hyperpolarisation activated, cyclic nucleotide gated) ion channel isoform HCN2 plays a central role in regulating the excitability of isolated nociceptive neurons. In intact mice, genetic deletion of HCN2 in sensory neurons which express the voltage-gated Na channel NaV1.8 abolishes both inflammatory and neuropathic pain. The present proposal will extend this work to investigate the following questions:
(i) HCN2 expression in NaV1.8-expressing neurons plays a special role in pain. What is the phenotype of these neurons, and what distinguishes them from other neurons not expressing NaV1.8?
(ii) Some mediator released following nerve lesion initiates neuropathic pain through an HCN2-dependent mechanism. What is the identity of this mediator, and through what intracellular signalling pathways does it act?
(iii) Do inhibitors of pathways elucidated under (ii) inhibit neuropathic pain?
(iv) How is expression of HCN2, both in the neuron as a whole and in its surface membrane, regulated by inflammatory mediators? Does the recently discovered auxiliary protein TRIP8b play a role in surface membrane expression?
(v) Is any role played by the HCN isoforms HCN3 and HCN4 in pain?

Who will benefit from this research?

The research is intended to be primarily curiosity-driven fundamental research which will elucidate the molecular mechanisms of inflammatory and neuropathic pain. The outcome is likely, however, to have considerable practical importance in the study of pain and in the development of novel analgesics. Work leading up to this application has identified HCN2 as a novel target for the control of inflammatory and neuropathic pain. The present proposal will further amplify and reinforce this work. In separate applications we are applying for funding to take this idea forward in collaboration with a company with facilities for high-throughput ion channel screening, with the objective of developing novel analgesics. If this work is successful the beneficiaries will be the many people who suffer from chronic inflammatory and neuropathic pain and whose pain is inadequately treated by currently available analgesics.

The general public has tremendous curiosity about pain, as is shown by the number of invitations I receive to give talks about my work and the general area of pain on radio and TV programmes, and in person to popular audiences. We are able to satisfy the natural curiosity of the general public, and they are therefore also beneficiaries of this work.

How will they benefit from this research?

The interest of the pharmaceutical industry in this area is shown by a current grant awarded to my lab from Organon, Inc (now Merck Sharp & Dohme) and the applicant has in the past had support from Merck Sharp & Dohme and from Glaxo SmithKline. An understanding of the involvement of ion channels such as HCN2 in pain is of particular interest to pharmaceutical companies.

Timescales for developing innovations arising from this research may not be long. An analogy can be drawn with the heat-sensitive ion channel TRPV1, which was published in 1997. Its importance in inflammatory pain was recognised as soon as genetically deleted mice were shown (in 2000) to exhibit an absence of inflammatory hyperalgesia. Pharmaceutical companies immediately began the development of antagonists and 10 years later a large number are in clinical trials, with several candidates well advanced into phase 2 trials. A similar rapid timescale of development has been seen with TRPA1, which was cloned in 2004 and for which several antagonists have already entered clinical trials with a view to treating bronchospasm in asthmatics. The analgesic effects of deletion of HCN2 are much more striking than either of these two examples, as in addition to a reduction of inflammatory pain, we have shown that deletion of this ino channel abolishes the muich more intractable neuropathic pain. We can envisage a similarly rapid timescale for the development of analgesics based on antagonists of HCN2.